What Your Eye Chart Measures and How It Works
Your eye chart test, also called a visual acuity test, measures how clearly you can see letters and symbols from a specific distance. The most common chart used in eye doctor offices is the Snellen chart, named after the Dutch ophthalmologist Herman Snellen who created it in 1862. This chart contains rows of letters that get progressively smaller as you move down the page.
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During your test, you'll sit about 20 feet away from the chart while an eye care professional points to different letters. You read the letters aloud, starting from the top (where letters are largest) and working downward. The eye doctor covers one eye at a time to test each eye separately. This allows them to measure vision in both eyes and identify any differences between them.
The chart works because of how vision works in the human eye. Light enters through your cornea and lens, which bend the light rays to focus them on the retina at the back of your eye. The retina sends signals through the optic nerve to your brain, which interprets these signals as images. When your eye focuses light correctly on the retina, you see sharp, clear images. When light focuses in front of or behind the retina, images appear blurry.
The standard notation you see on your results, like 20/20, compares your vision to what a person with normal vision should see. The first number (20) represents the distance at which you stand from the chart—20 feet. The second number (20) represents the distance at which a person with typical vision would need to stand to see the same line of letters clearly. So 20/40 vision means you must stand 20 feet away to see what someone with normal vision can see from 40 feet away. If your result is 20/15, you have better-than-average vision and can see details from 20 feet that most people need to be 15 feet away to see.
Practical Takeaway: Understanding your visual acuity number helps you know how your eyesight compares to a standard baseline. Bring your previous test results to your appointments so your eye care provider can track whether your vision has changed over time.
Understanding Your Prescription and What the Numbers Mean
After determining your visual acuity, your eye care provider uses additional equipment and tests to measure your prescription. Your prescription contains specific numbers that describe exactly how your eyes focus light. These numbers look like a code at first, but each one describes something different about how your eyes work.
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The prescription typically includes three main measurements for each eye. The first number, called the sphere (abbreviated as SPH), measures whether you are nearsighted or farsighted. Nearsighted people see objects close to them clearly but struggle to see distant objects. Farsighted people see distant objects clearly but may have trouble focusing on nearby objects. Sphere measurements use positive numbers for farsightedness and negative numbers for nearsightedness. For example, -2.00 means you are moderately nearsighted, while +1.50 means you are mildly farsighted.
The second number, called cylinder (abbreviated as CYL), measures astigmatism—an irregular shape of your cornea or lens that causes blurred vision at all distances. Not everyone has astigmatism, so this number might be blank or show 0.00 on your prescription. Like the sphere measurement, cylinder uses positive and negative numbers. The third number, called axis (abbreviated as AXIS), works with the cylinder number and describes the direction of the astigmatism, measured in degrees from 0 to 180.
Many prescriptions also include a fourth measurement for people over 40 or those who have trouble focusing on close objects. This is called addition (abbreviated as ADD) and measures the extra focusing power needed for reading and close work. You might see this on bifocal or progressive lens prescriptions. Additionally, your prescription may include a measurement called prism, which helps correct eye alignment problems, though most people don't have this measurement.
When you look at your prescription, you might see something like: OD (right eye) -1.50 -0.50 x 180 ADD +2.00. This means your right eye is somewhat nearsighted, has mild astigmatism, and needs additional power for reading. Your left eye (OS) would have its own separate measurements. Prescriptions usually remain valid for one to two years, though some states allow three-year validity periods.
Practical Takeaway: Write down your prescription numbers and keep them in a safe place. You can use this information to order glasses or contacts from various retailers, not just your eye doctor's office. Ask your provider to explain each number during your visit so you understand what they mean for your specific vision needs.
How Eye Care Providers Test for Common Vision Problems
Beyond the basic eye chart and refraction test that measures your prescription, eye care providers perform several other tests during a comprehensive eye examination. Understanding what these tests measure helps you know why your eye doctor performs them and what information they gather about your eye health.
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The tonometry test measures eye pressure, which is important for detecting glaucoma. During this test, the eye care provider gently places a small probe against your eye's surface or blows a puff of air at your eye to measure internal eye pressure. Normal eye pressure ranges from 10 to 21 millimeters of mercury (mmHg). Higher pressure can damage the optic nerve over time, even without causing symptoms. Glaucoma affects approximately 3 million Americans, but many don't know they have it because early stages often show no warning signs.
The visual field test, also called perimetry, measures how much of your side vision (peripheral vision) you have. You'll sit at a machine and look straight ahead at a central point while lights flash around your peripheral vision. You press a button each time you see a light. This test helps detect glaucoma, retinal problems, and some neurological conditions. Your visual field should be roughly 170 degrees horizontally and 135 degrees vertically in normal eyes, though most people don't see this entire range clearly.
The pupil dilation test involves placing special drops in your eyes to make your pupils larger. This allows your eye care provider to examine the retina and optic nerve more thoroughly. During this test, you might see floaters—small spots or cobweb-like shapes that drift across your vision. These are typically harmless shadows cast by tiny structures inside the eye, but the eye doctor checks to ensure nothing serious is causing them. After dilation, your vision may remain blurry and you'll be sensitive to light for several hours, so many people bring sunglasses to their appointments.
Color blindness tests, like the Ishihara test, check whether you can distinguish different colors normally. The test shows a series of colored dots arranged in patterns. People with color blindness may not see certain numbers or patterns in these dots. About 8% of men and 0.5% of women have some form of color blindness, which is usually hereditary. The Amsler grid is another common test that checks for problems with your central vision and can detect early signs of macular degeneration, a condition that can lead to vision loss in older adults.
Practical Takeaway: Ask your eye care provider which tests they're performing and why. These tests provide important information about your eye health beyond just whether you need glasses. If your provider recommends additional testing, ask what they're checking for and when you should follow up with results.
What Different Eye Chart Results Tell You About Your Vision
Your eye chart results are just one part of your overall eye health picture, but they do tell you important information about how your eyes focus light. Most people in the United States have refractive errors, which means their eyes don't focus light exactly on the retina. According to the National Eye Institute, about 42% of Americans between ages 18 and 60 have myopia (nearsightedness), and the rate has been increasing over the past few decades.
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If your results show 20/20 vision, this means your eyes focus light correctly on your retina, and you see standard details at the standard distance. However, 20/20 doesn't mean you have perfect vision—it only means you have typical visual clarity. You could still have other eye health issues that don't affect visual acuity, such as early cataracts, eye pressure problems, or retinal diseases. This is why eye care providers perform additional tests beyond the eye chart.
Results better than 20